Is a student who vowed to slaughter an animal for his colleagues due to his success in final exams obligated to fulfill his vow immediately, and is it permissible for him to offer expiation instead of slaughtering?

Whoever vows to slaughter an animal if he passes examinations, it is obligatory for him to fulfill his vow immediately once the condition is met, because it is a vow of obedience. It is not permissible to delay fulfilling the vow if one is able to, but if one is unable, he should do it whenever he can. Expiation is not sufficient for slaughter in a vow of obedience. Rather, expiation is for vows of obstinacy and anger or vows concerning permissible matters, where one is given the choice between fulfilling the act or offering an expiation for an oath.